To evaluate a possible role for colony stimulating factor-1 (CSF-1) in human ovarian function, the peripheral blood CSF-1 concentration throughout the human menstrual cycle and during ovarian stimulation was monitored. Blood was sampled across the menstrual cycle (n = 10) and at specific times during ovarian stimulation. In addition, the CSF-1 concentrations in follicular fluid (FF) during the follicular phase and during the luteinizing hormone (LH) surge of natural cycles, as well as 35-37 h after human chorionic gonadotrophin (HCG) during ovarian stimulation, were determined. There was no significant variation in CSF-1 concentrations during the natural menstrual cycle (median 470, range 212-1364 pg/ml). CSF-1 concentrations in FF (n = 11) were about four-fold higher (P < 0. 0001) than those in plasma of the same patients. CSF-1 concentrations in these FF showed some stage dependent variability, with significantly higher values during the ovulatory phase (median of 2017 pg/ml, range 1131-2236 pg/ml), compared to mid-follicular phase (median 961 pg/ml, range 830-1340 pg/ml; P = 0.02). During ovarian stimulation (n = 20), the plasma concentrations were similar to a time prior to stimulation up to and including 35-37 h after HCG. On day 9 after HCG, the values (median 644, range 357-1352 pg/ml) were significantly higher compared to pre-stimulation (median 422, range 253-1598 pg/ml; P < 0.05) and 35-37 h after HCG (median 458, range 250-658 pg/ml; P < 0.01). FF concentrations (n = 27) of CSF-1 at oocyte retrieval (median 3116, range 1824-5883 pg/ml) were about seven-fold higher than blood concentrations (median 472, range 250-1055 pg/ml; P < 0.0001). These results suggest that the intra-ovarian CSF-1, possibly induced by LH/HCG, plays an important role during ovulation and luteinization.